This is one of the difficulties I encountered when I was making an R script for a sampling task in a project. First I thought this would be a simple problem, and apply(x, 2, sort) might be OK. Later I found it was wrong:
> a
[,1] [,2]
[1,] -14 11
[2,] 1 5
[3,] 1 2
[4,] 1 3
[5,] -2 -20
> apply(a,2,sort)
[,1] [,2]
[1,] -14 -20
[2,] -2 2
[3,] 1 3
[4,] 1 5
[5,] 1 11
Apparently the data structure was destroyed. I want to sort the matrix by a column (or several columns), but apply(x, 2, sort) sorts all the columns respectively. Of course a simple function sort() will return a more “terrible” result.
The correct way is to use the function order(), which returns a permutation which rearranges its first argument into ascending or descending order, but order() alone still can’t solve the problem. Just compare
> a[order(a[, 1]), ]
[,1] [,2]
[1,] -14 11
[2,] -2 -20
[3,] 1 5
[4,] 1 2
[5,] 1 3
with
> a[order(a[ ,1], a[ ,2]), ]
[,1] [,2]
[1,] -14 11
[2,] -2 -20
[3,] 1 2
[4,] 1 3
[5,] 1 5
Today I looked up for this question in “R FAQ” and easily found it. It seems that I still lack the habit of searching the R site.
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